The present invention relates to a grinding method, in particular, the invention relates to the grinding of substantially cylindrical paper machine rollers, but it can also be used in grinding the surfaces of other elongated cylindrical or conical objects. In the following, however, the invention will be mainly described by referring to cylindrical paper machine rollers.
Heavy and relatively long paper machine rollers are generally stored by suspending them by their hubs, with the result that the rollers are bent by the action of their own weight, i.e. curved downward relative to the straight line between their hubs. In practice, the roller hardly ever bends in one plane only; instead, due to a variation of stiffness and different positions during transport and storage, the ultimate deflection is a three-dimensional curve. When the roller is again mounted as a rotating component, it begins to straighten out slowly, approaching the straight line between its hubs. The speed and degree of straightening is proportional to the length of time the roller has been kept in a non-rotating condition in storage. Typically, this time varies from a few hours to several days.
When a grinding operation on a curved roller taken from storage is started, the deflection causes unnecessary working time expenses because the roller has to be rotated in the grinding machine for several hours before actual grinding can be started. By the time grinding is started, the roller is seldom completely straight, which is why the grinding pressure and also the amount of material removed vary depending on the degree of eccentricity of the roller. Thus, the roller undergoes unnecessary machining and material is removed from where it should not in order to achieve a desired rotational and length profile of the roller.
If the roller has originally been circular in section but undergone deflection during storage, it may have been ground to a non-circular cross-sectional form while expecting it to be straightened. If the roller is to be given a circular cross-sectional form, then it has to be ground long enough to allow the rotation to produce sufficient straightening or a sufficiently stable condition, and after this the grinding has to be continued until the deformation produced by incorrect grinding has been abraded.
After the roller has been ground, it is often put back in store, where the bending process starts again. When finally mounted in the paper machine, the roller may be as curved as before the grinding.
The object of the invention is to eliminate the above-mentioned drawbacks. A specific object of the invention is to disclose a new type of grinding method whereby the currently most widely used single-disc machines can achieve the same grinding precision that has previously only been attainable by using dual-disc machines with floating stone suspension.
In the text of the present application, the roller form aimed at is described by the general term xe2x80x98circularxe2x80x99 to refer to the most advantageous rotational profile of a roller. The desired grinding profile of the roller may differ from a circular orbit if grinding is undertaken to compensate for errors arising in a grinding machine or paper machine from e.g. variations in roller stiffness.
In the invention, it has been discovered that the effect of the deflection of a roller can be compensated by using a suitable control system. The grinding stone can be made to follow the target circumference of the roller in addition to a desired length profile, so that only the non-circularity and other surface deformations of the roller are ground off. By taking the form of the deflection and its straightening during grinding into account, the roundness of the roller can be restored in the shortest possible time.